My 6-year-old recently went to the dentist to get a cavity filled. Basically, it seems that she broke out in hives a couple minutes after inhaling it though she's had the gas before with seemingly no ill effects.

Background. She was not on any medication except for some homeopathic pills called BrightSparks which is used for things like better focus. She was on the gas for a couple minutes at most before she started squirming too much due to the distasteful smell. I went in to talk to her and noticed a couple of bright red patches on her lower cheeks. Within 5 minutes the patches grew, filled with hives (bumps) and became itchy. It's possible but seemed that it was not an allergic reaction to the latex connected to the mask because the patches were larger than the area the latex could have touched (beneath her jawbone). At first I thought the red patches were a result of her agitation (crying) but quickly realized that was not the case.

She had also had a fever the two previous nights (no fever during the day) and during each day following, had a single "hive" under her eye when she started crying. It didn't get any larger or spread beyond the single bump.

I read on the Web that L-Argining, present in Nitrous Oxide, does cause hives. Can anyone confirm or refute this? What is the dentist's other pain remediation options if she is allergic to the NO?